This invention provides a method and apparatus for removing particulates, including submicron particle size insoluble solid particulates, from a gas stream using a fiber bed collector from which collected insoluble solid particulates can be removed during operation without the need for removing the fiber bed from service for cleaning. The invention permits high collection efficiency, extended continuous operation without any unacceptable increase in pressure drop and service life for the fiber bed separator not limited by the pluggage rate of the fiber bed.
Industrial waste gases and process gas stream frequently contain undesirable solid particulates which must be separated from the gas stream for environmental or process requirements. Particulates having a particle size of 3 or more microns are easily recovered with high efficiency in many types of conventional separators. Smaller particulates, and particularly those of submicron size, and especially those down to about 0.2 micron in size are more difficult to separate with a high degree of efficiency.
Fiber bed separators are commonly used for particulate separation. These fall into three classes: high efficiency as regards submicron particulates, high velocity, and spray catchers.
High efficiency fiber bed separators typically use 5 to 20 micron fibers, such as glass fibers, packed to a bed voidage of from 85 to 98% for separation of submicron particulates with 95 to 98% efficiency, but at low gas flow rates (i.e., bed velocities) typically up to about 0.5 feet per second (or about 0.15 meters per second). At be velocities about this level, significant increases in pressure drop through the fiber bed will result, with consequent increase in power requirements. Liquid particulates such as mists, etc. effectively drain from the fiber bed during operation. Readily soluble solid particulates can be removed from such fiber beds during operation by irrigating the fiber bed with a liquid in which the solid particulates are readily dissolved. Gas streams containing any appreciable loading of insoluble solid particulates, and particularly over 1 micron in size, cannot be treated in high efficiency fiber bed separators, even with irrigation, without gradual build-up of insoluble particulates within the fiber bed until the fiber bed either becomes plugged or the pressure drop through the fiber bed due to plugging has increased to an unacceptable degree, at which point the fiber bed must be taken out of service for cleaning or replacement.
High velocity fiber bed separators are used in applications where high gas flow rates (e.g., bed velocities of from 5 to 10 feed per second; i.e., 1.5 to 3 meters per second) are desired and less efficient separation of submicron particulates can be tolerated. This type of fiber bed separator typically used 25 to 50 micron diameter fibers with a bed voidage of from 85 to 98%. For any given bed voidage, the coarser fibers of a high velocity fiber bed (as compared to a high efficiency fiber bed) has considerably less fiber surface area per unit volume of fiber bed and accordingly a more open network of fibers. Therefore, this type of bed relies more heavily on an inertial impaction mechanism of particulate separation and thus has poorer efficiency for separation of submicron particulates. Removal of insoluble solid particulates from such fiber beds by irrigation or flushing is possible for lower particulate loadings and particulates up to about 2 microns in size, but here again gradual increase in pressure drop and eventual pluggage will result due to difficulty in removing larger sized particulates present in most applications, requiring removal from service for cleaning or replacement after several days to weeks of operation.
Finally, spray catcher fiber beds are used in applications wherein high volumes of gases are to be treated and separation of only large particulates of 3 microns or greater in size is of concern. A spray catcher typically uses fibers of about 100 to 300 microns average diameter with a bed voidage of 90 to 98%. These fiber beds have the lowest fiber surface area per unit volume of any of the types of fiber bed separators discussed herein at any given bed voidage. They rely almost entirely on the inertial impaction separation mechanism. Accordingly, the spray catcher has the poorest efficiency of all for separation of submicron particulates. However, even large (e.g., 5 micron) insoluble solid particulates can be irrigated or flushed out of spray catcher fiber beds.
This invention is directed to the use of the spray catcher type of fiber bed for the separation of particulates, and particularly insoluble solid particulates, from gas streams at high bed velocities of about 5 feet per second (i.e., 1.8 meters per second) or more, with a high efficiency of at least 90% on separation of submicron particulates, and with liquid irrigation of the fiber bed to remove the collected solid particulates therefrom for long term continuous operation, without the need for taking the fiber bed out of service for cleaning or replacement due to pluggage.
As used herein, the term "insoluble solid particulates" refers to solid particulates which will not dissolve in water or such other liquid system as may be selected as the irrigation liquid, or which have such low solubility rates in the liquid that their solubility cannot be effectively used for their removal from the fiber bed.